1. Field of the Invention
The present invention relates to a measuring method and apparatus of a roof surface of an optical element, such as a pentagonal roof prism used for example in a single lens reflex camera.
2 Description of Related Art
In a view finder of a single lens reflex camera, an image, formed on a focusing screen through a taking lens and a mirror, is converted to an erect image by a pentagonal roof prism allowing observation to be observed through an eyepiece. Therefore, if the pentagonal roof prism is not precisely assembled or manufactured, an unclear double image, for example, may appear in the view finder. This is caused particularly when the roof angle of the pentagonal roof prism is inaccurate or the edge of the roof surface is rounded. To avoid this, it is necessary to detect or measure an error in the angle of the roof surface (roof angle) and also to determine whether or not the roundness at the apex of the roof surface is within an allowable limit.
The detection requires that; 1 the measurement can be quantified, 2 the roof angle and the roundness can be independently or simultaneously detected, 3 the detection can be effected within a short period of time, and 4 the measurement is not adversely affected by a mechanical oscillation (external oscillation), etc., in addition to general requirements needed for common measurements.